1. Field of the Invention
The present invention relates to a resin-encapsulated semiconductor device in which a semiconductor element is encapsulated with a resin for the purpose of protecting the semiconductor element and, more particularly, to a technique suitably applicable to a resin-encapsulated LSI package having a structure in which leads are connected by tiebars.
2. Description of the Related Art
The forms of LSI packages are roughly classified into surface mounting type packages such as a BGA (Ball Grid Array) and a CSP (Chip Size Package) in which electrodes are arranged in the form of a grid on the entire surface of a substrate on which a semiconductor chip is mounted, and edge mounting type packages such as an SOP (Small Outline L-Leaded Package), a QFP (Quad Flat Package), and a TSOP (Thin Small Outline Package) in which electrodes are extracted from the edge of a semiconductor chip by using bonding wires and a lead frame. The former packages are used in some restricted logic products requiring an increase of the number of pins. That is, the latter edge mounting type packages are used in most LSI packages represented by a DRAM.
A general edge mounting type package will be described below.
FIG. 8 is a plan view showing an SOP as a representative edge mounting type package before resin encapsulation. The manufacturing process of this SOP will be described below with reference to FIG. 9.
First, the lower surface of a semiconductor chip 1 is bonded to the upper surface of a die pad 2 in a lead frame 12 including the die pad 2, inner leads 4, outer leads 5, and tiebars 8 (step S101). Bonding pads 13 formed on the semiconductor chip 1 are electrically connected to the inner leads 4 by bonding wires 3 (step S102).
Subsequently, the semiconductor chip 1 and the bonding wires 3 are encapsulated with an encapsulating resin 18 by transfer molding (step S103). The resin extending from the edge of the package is removed by punching by a resin cutting punch (step S104). The tiebars 8 in the lead frame 12 are cut to electrically separate the outer leads 5 (step S105).
Subsequently, the resin burr not removed by the resin cutting and the tiebar cutting is removed by water-jet (step S106), the outer leads 5 are cut from a base portion 14 (step S107), and the leads are plated with solder (step S108).
Finally, the leads are formed into a desired shape, thereby completing the SOP (step S109).
In the manufacturing process of this SOP, the deflashing step of removing the resin burr extending from the edge of the package is provided after the semiconductor chip 1 is encapsulated with the encapsulating resin 18. This extends the manufacturing period or increases the manufacturing cost. To eliminate this deflashing step, it is only necessary to prevent the resin from extending from the edge of the package. The easiest method of realizing this is to arrange the tiebars 8 close to a prospective resin encapsulation region 20. Ideally, it is desirable that the tiebars 8 are previously arranged along the prospective resin encapsulation region 20 in the lead frame 12.
In the manufacturing process of the SOP as described above, however, the position of the molding die for resin encapsulation may deviate from the prospective position on the basis of the lead frame 12 when the semiconductor chip 1 and the bonding wires 3 are encapsulated with the encapsulating resin 18 by transfer molding. Therefore, in consideration of the amount of this positional deviation, the positions of the tiebars 8 in the lead frame 12 must be determined. If such a positional deviation occurs when the tiebars 8 are arranged along the prospective resin encapsulation region 20, as shown in FIG. 10A, a tiebar cutting punch 9 punches not only the tiebar 8 but also the encapsulating resin 18 to break the package in tiebar cutting. In some cases, as shown in FIG. 10B, the tiebar 8 is partially resin-encapsulated and can not be cut.
Detailed steps in the molding step in which the semiconductor chip 1 and the bonding wires 3 are encapsulated with the encapsulating resin 18 by transfer molding will be described below in order to clarify the amount of the positional deviation between the prospective resin encapsulation region 20 and the actual region of the complete package.
FIGS. 11A to 11D are sectional views, taken along a line VI--VI in FIG. 8, showing steps in encapsulating the semiconductor chip 1 and the bonding wires 3 with the encapsulating resin 18 in order of steps.
First, as shown in FIG. 11A, the lead frame 12 on which the semiconductor chip 1 is mounted is placed on a lower mold 22 of a molding die for resin encapsulation. More specifically, the lead frame 12 and the lower mold 22 are matched by fitting positioning pins 23 of the lower mold 22 into sprocket holes 15 formed in the lead frame 12. The processing accuracy of the sprocket holes 15, the positioning pins 23, and the lower mold 22 is about a few .mu.m. The positioning margin between the sprocket hole 15 and the positioning pin 23 is about 20 to 30 .mu.m.
Subsequently, as shown in FIG. 11B, an upper mold 24 of the molding die for resin encapsulation is moved down from above the lead frame 12 to cover the lower mold 22 and the molding die is clamped. In this case, the upper and lower molds 24 and 22 are matched by fitting the positioning pins 23 of the lower mold 22 into positioning recesses 25 formed in the upper mold 24. That is, the positional relation between the upper mold 24 and the lead frame 12 are not directly determined. The processing accuracy of the upper mold 24 is about a few .mu.m. The positioning margin between the positioning pin 23 and the positioning recess 25 is about 50 .mu.m.
Subsequently, a resin is injected into a cavity of the molding die through an opening formed in the upper mold 24. Thereafter, as shown in FIG. 11C, the opening is closed with a plunger 26, and the encapsulating resin 18 is set in a pressure.
Finally, as shown in FIG. 11D, the upper and lower molds 24 and 22 and the plunger 26 are taken off the encapsulating resin 18, thereby finishing the molding.
In the conventional molding die for resin encapsulation, as shown in FIG. 11B, a width E101 of a cavity of the upper mold 24 of the molding die is made equal to a width E102 of a cavity of the lower mold 22 of the molding die. Therefore, when the upper and lower molds 24 and 22 are matched, there is a possibility that the positions of the upper and lower molds 24 and 22 deviate from the prospective resin encapsulation region 20 respectively. More specifically, after molding, the actual package region can horizontally deviate from the prospective package region on the basis of the lead frame 12 by about 100 .mu.m which is equivalent to the sum of the processing accuracy of the components and the positioning margin described above.
Also, the length of the edge of the tiebar cutting punch 9 for cutting the tiebars 8, in a direction perpendicular to the tiebars 8 in a plane horizontal to the lead frame 12, is generally made larger by about 70 .mu.m than the length of the tiebars 8 in the same direction in order to reliably cut the tiebars 8.
That is, in the stage of designing the lead frame, it is necessary to ensure a distance of at least about 170 .mu.m between the tiebars 8 and the prospective resin encapsulation region 20.
When a package is manufactured by using the lead frame 12 in which the tiebars 8 are arranged at a distance of 170 .mu.m from the prospective resin encapsulation region 20, the resin extends by about 170 .mu.m at its longest from one side of the package to form resin burr. The amount of remaining resin burr on one side of the package is defined to be 150 .mu.m or less in a dimension standardized by EIAJ, although it depends upon the type of product. Accordingly, even when the tiebars 8 are arranged closest to the prospective resin encapsulation region 20, removal of resin burr must be performed after tiebar cutting in the conventional methods.